1. Field of the Invention
The present invention relates to a method for determining a focusing control operation position in an opto-magnetic disc recording/reproducing apparatus. More particularly, the present invention relates to a method for determining a focusing control operation position in an opto-magnetic disc recording/reproducing apparatus for shortening a focusing time required for determining a focusing control operation position of an objective lens and preventing deterioration of a focusing coil.
2. Description of the Prior Art
Currently, opto-magnetic discs have been developed to be used as one kind of optical discs to/from which information can be recorded and/or reproduced. The opto-magnetic disc is loaded to be used in a mini-disc player which is an opto-magnetic disc recording/reproducing apparatus. In the mini-disc player, since a small-sized mini disc, e.g., having a diameter of 64 [mm], is retentively loaded within a cartridge having dimensions of 68 mm.times.72 mm.times.5 mm, it is handy to carry the disc, and a user can use two types of recording media such as an optical disc exclusive to reproducing and the opto-magnetic disc to/from which information can be recorded and/or reproduced. In addition, the mini-disc player executes a random access function like a compact disc player and can execute 74-minute reproducing operation, by a data compression method, even when a small-sized disc is loaded therein as if a compact disc were loaded therein, thereby having been widespread.
FIG. 1 is a sectional view for showing a sectional structure of a general opto-magnetic disc. As shown in FIG. 1, an opto-magnetic disc 10 includes a transparent substrate 11 formed of a polycarbonate particularly among transparent synthetic resins such as a PVC, PMMA and polycarbonate so as to allow laser beam for reading out audio/video signals to pass therethrough. Transparent substrate 11 has an uneven structure which forms pits corresponding to predetermined optical signals. Accordingly, a reflective layer 12 is formed of a thin film made of metal, i.e., aluminum, having a large reflection factor in order to reflect the optical signal. Also, a protective layer 13 made of a hard synthetic resin is adhesively formed above reflective layer 12 in order to protect reflective layer 12. A first dielectric layer 14 is adhesively formed below reflective layer 12. A second dielectric layer 15 is adhesively formed above transparent substrate 11. A magnetic layer 16 is adhesively formed between first dielectric layer 14 and second dielectric layer 15.
FIG. 2A is a schematic view for illustrating a recording operation in a recording/reproducing apparatus for the opto-magnetic disc shown in FIG. 1. FIG. 2B is a waveform for representing characteristics of signal recording current applied to a magnetic head in order to record information on a recording surface of the opto-magnetic disc by means of the magnetic head shown in FIG. 2A. As shown in FIGS. 2A and 2B, a principle in which information is recorded on opto-magnetic disc 10 having the above-mentioned structure, is as follows. Laser beam is irradiated onto a lower outer surface of opto-magnetic disc 10 by using a laser apparatus (not shown) having an output power of approximately 4 [mW]. When a temperature at a portion (hereinafter referred to as "beam spot portion") 17 of the lower outer surface of opto-magnetic disc 10 to which the laser beam is applied, is increased to approximately 180.degree. C., a head driving signal 281, i.e., signal recording current, begins flowing through a magnetic head coil 261, in accordance with a direction in which required information is recorded on opto-magnetic disc 10, included in a magnetic head 260 placed on an upper outer surface of opto-magnetic disc 10. At this time, a position of beam spot portion 17 is moved while opto-magnetic disc 10 is rotated, so that beam spot portion 17 is cooled and has a magnetism by means of polarity of magnetic head 260. Namely, while head driving signal 281 is supplied to magnetic head coil 261, a track surface of opto-magnetic disc 10 is polarized to N and S.
A principle in which information is reproduced from opto-magnetic disc 10, is as follows. Laser beam is irradiated onto the lower outer surface of opto-magnetic disc 10 by using a laser apparatus (not shown) having an output power of approximately 0.6 [mW]. In terms of the Kerr effect, in accordance with the N and S polarities of polarized opto-magnetic disc 10, the laser beam incident onto opto-magnetic disc 10 is reflected with the state that a polarizing plane thereof is rotated clockwise or counter-clockwise by approximately 0.2.degree..about.0.3.degree. C., and passes through a polarizing beam splitter (not shown) to be incident to the set of two light receiving devices (not shown) which are installed with a predetermined gap between them. At this time, signals having antiphase are generated from the light receiving devices. Then, on the basis of the antiphase signal, the N pole and S pole of a magnetic material included in opto-magnetic disc 10 are respectively converted into voltage values of "high" and "low" levels in order to be recognized as data.
As described above, in order to accurately execute the operation in which data is read out from opto-magnetic disc 10, the laser beam should be accurately focused on reflective layer 12. Generally, the reflectance of protective layer 13 and refective layer 12 included in opto-magnetic disc 10 are not considerably different from each other in the opto-magnetic disc recording/reproducing apparatus. Therefore, a time (hereinafter. referred to as "set time of timer") required to focus at a desired position of protective layer 13 of opto-magnetic disc 10 as shown in FIG. 31), is sufficiently set, and amount of the focusing control current flowing through the focusing coil is increased, thereby upwardly moving objective lens 242 in order to execute the focusing control operation on reflective layer 12. After that, when set time of timer 800 is terminated, objective lens 242 (shown in FIG. 4) gradually moves downwards while the amount of the focusing control current is decreased, thereby executing the focusing operation on refective layer 12.
FIG. 3A is waveform for illustrating characteristics of a focusing error signal. FIG. 3B is a waveform for illustrating characteristics of a light receiving device output signal provided by a light receiving device. FIG. 3C is a waveform for illustrating characteristics of a focusing detecting signal. FIG. 3D is a waveform for illustrating characteristics of a focusing detecting signal. FIG. 3D is a waveform for illustrating characteristics of a focus zero crossing signal. FIG. 4 is a block diagram for showing a servo system of a recording/reproducing apparatus for the opto-magnetic disc shown in FIG. 1. Namely, if the magnitude of the focusing control current is increased at the begining of the focusing operation, i.e., during the set time of the timer, objective lens 242 moves upwards in order to execute a focus searching operation. Accordingly, as shown in FIGS. 3A to 4, an arbitrary focusing error signal 400 (shown in FIG. 3A) is detected by a focusing error detecting section (not shown) included in a focusing servo section 300. A light receiving device output signal 247 provided by a light receiving device 241 in order to detect focusing error signal 400, has the waveform as shown in FIG. 3B.
Focusing detecting signal 302 as shown FIG. 3C denotes a signal detected by focusing servo section 300 when receiving a first signal of quantity of light 500A corresponding to an inaccurate focusing on a device or a second signal of quantity of light 500B corresponding to an accurate focusing on a device. e.g., light receiving device 241 divided-by-four. A signal of quantity of light 500 denotes first signal of quantity of light 500A or second signal of quantity of light 500B. Focus zero crossing signal 700 shown in FIG. 3D is a signal detected by a focus zero crossing switch section (not shown), included in a focusing servo section 300, corresponding to "ON" state of a focusing servo section 300. When focusing detecting signal 302 is applied to a control section 140 (shown in FIG. 4), focus zero crossing signal 700 received within set time of timer 800 is disregarded in case that set time of timer 800 is not terminated.
Thereafter, in accordance with a continuous ascending operation of objective lens 242, when the laser beam deviates from reflective layer 12 included in opto-magnetic disc 10 to focus on protective layer 13 adhesively disposed above reflective layer 12 (namely, when set time of timer 800 is terminated), a value of a focusing control current signal 301 flowing through a focusing coil 243 is decreased under the control operation of control section 140, and a focusing position of objective lens 242 is lowered. As a result, when the laser beam is focused on reflective layer 12, light receiving device 241 receives second signal of quantity of light 500B corresponding to the accurate focusing. Consequently, control section 140 is supplied with both focusing detecting signal 302 provided by focusing servo section 300 and focus zero crossing signal 700 corresponding to "ON" state of focusing servo section 300. Then, under the control operation of control section 140, the focusing operation is continuously carried out from the point when focus zero crossing signal 700 is generated.
As described above, for determining a desired focusing position in the conventional opto-magnetic disc recording/reproducing apparatus, the laser beam is primarily focused on a predetermined position of protective layer 12 included in opto-magnetic disc 10. Then, the focusing position of objective lens 242 is gradually lowered until reaching reflective layer 12, and the focusing is in "ON" state at this position. Therefore, in the aforementioned method for determining the focusing control operation position, it takes unnecessarily much time, and the laser beam should be focused onto the predetermined position of protective layer 13 included in opto-magnetic disc 10, causing excess current to flow through the focusing coil. Furthermore, as the focusing control current is frequently applied to the focusing coil, the characteristics of the focusing coil are deteriorated, so that the accurate focusing control operation is hard to be executed.